Int. J. Environ. Res., 7(3):581-594,Summer 2013ISSN: 1735-6865

Received 19 Nov. 2012; Revised 18 Feb. 2013; Accepted 2 March 2013

*Corresponding author E-mail:mgav@tuiasi.ro

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Bioremediation of Cr(VI) Polluted Wastewaters by Sorption on HeatInactivated Saccharomyces cerevisiae Biomass

Hlihor, R.M.1, Diaconu, M.1, Fertu, D.1, Chelaru, C.2, Sandu, I.3, Tavares, T.4 and Gavrilescu, M.1*

1Gheorghe Asachi” Technical University of Iasi, Romania, Department of EnvironmentalEngineering and Management, 73 D. Mangeron Street, 700050 Iasi, Romania

2INCDTP Division – Leather and Footwear Research Institute, Bucharest, Romania3Alexandru Ioan Cuza” University of Iasi, 11 Carol I Blvd., 700506 Iasi, Romania

4University of Minho, Department of Biological Engineering, Campus de Gualtar, 4710-057,Braga, Portugal

ABSTRACT: The potential of heat inactivated Saccharomyces cerevisiae in the bioremoval and reduction ofCr (VI) ions from wastewaters was evaluated in terms of metal uptake in time and at equilibrium, andbiosorption efficiency, by varying pH, biosorbent doses, contact time and temperature, in batch mode. Duringthe sorption process, the heat inactivated biomass of the yeast Saccharomyces cerevisiae is capable ofreducing Cr(VI) to Cr(III). Different kinetic models based on adsorption and reduction are used to representthe kinetic data of Cr(VI) bioremoval by S. cerevisiae, in explaining the biosorption mechanism of heavymetals and potential rate-controlling steps, in the perspective of full-scale process design. The results indicatedsome potential differences in the Cr(VI) removal mechanism at different experimental conditions. FTIR andSEM analysis were performed as well as to elucidate the mechanism of metal bioremoval by S. cerevisiae.FTIR spectra indicate that heavy metal bioremoval process doesn’t imply in this case the formation of stablecovalent bonds, but it is predominantly based on chemical interactions, ion-exchange type. The SEM micrographsof Cr-loaded yeast, indicates that the surface morphology doesn’t change much after chromium ions wereuptaken. This leads to the conclusion that Cr(VI) reduction occurs at the interface of the adsorbent.

Key words: Biosorption, Cr(VI) reduction, Yeast biomass, Kinetics, Sorption mechanism

INTRODUCTIONThe increase in environmental contamination is a

major consequence of industrial development, with aparticular emphasis on pollution with heavy metals.Unlike many organic pollutants, which eventuallydegrade to carbon dioxide and water, most metals arenot destroyed, but they are accumulating in theenvironment, especially in lake, estuarine, or marinesediments at an accelerated pace (Sarkar, 2002; Cvijovicet al., 2011; Cozma et al., 2010). Metals can also betransported from one environmental compartment toanother, which complicates the containment andtreatment problems (Jackson et al., 2010; Pavel et al.,2010; Wuana and Okieimen, 2011). Consequently, heavymetals are closely connected with environmentaldeterioration and the quality of human life, and thushave aroused concern all over the world (Hlihor andGavrilescu, 2009; Ho and El-Khaiary, 2009; Gradinaru et

al., 2010). The demand for cleanup of contaminatedenvironmental compartments to meet regulatory limits,as well as the increasing value of some metals place acall for efficient and low-cost treatment and metalrecuperation technologies. Conventional methods forheavy metals removal from industrial effluents includeprecipitation, ion exchange, electrochemical methodsand reverse osmosis (Kurniawan et al., 2006; Popa etal., 2010; Hong and Ning, 2011; Katsou et al., 2011).Chromium(VI), one of Cr oxidation states is asuspected carcinogen and a potential soil, surfacewater, and groundwater contaminant. Although Cr(VI)may occur in the natural environment, human-causedCr(VI) contamination has recently been the focus ofmuch scientific discussion, regulatory and legalconcerns. Chromium (Cr) is widely used in surfacetreatments, stainless steel and alloy production,pigments, leather processing, production of catalysts

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(Singh and Gadi, 2009; Dunea and Iordache, 2011;Cvijovic et al., 2011). Although there exist alreadyestablished physico-chemical methods and processesfor chromium removal from the environment, theirapplication is sometimes restr icted, due totechnological or economical constrains, biosorptioncan represent a sustainable alternative for metalremoval and/or recovery, based on metal-sequesteringproperties of dead or living biomass. The majoradvantages of biosorption technology are itseffectiveness in reducing the concentration of heavymetal ions to very low levels and the use of inexpensivebiosorbent materials (Volesky, 2007; Gadd, 2009; Wangand Chen; 2009; Owlad et al., 2009; Chojnacka, 2010;Figueiredo et al., 2010a, Figueiredo et al., 2010b;Gavrilescu, 2010; Kicsi et al., 2010; Pilli et al., 2010;Ramesh et al., 2011; Zhang et al., 2011).

Many biological materials can bind heavy metals,but only those with sufficiently high binding capacityand selectivity for heavy metals are suitable for use ina full-scale biosorption and bioaccumulation processes(Gavrilescu, 2004; Gadd, 2009; Wang and Chen; 2009;Karaduman et al., 2011; Gomes et al., 2011). Biosorptionnot only offers an innovative alternative to otherremediation approaches, but it also allows metalsrecovery (Kotrba, 2011; Viraraghavan and Srinivasan,2011). Among the different types of biomass proposedfor heavy metals bioremediation, yeast cells ofSaccharomyces cerevisiae, seems to be a promisingalternative (Cojocaru et al., 2009; Machado et al., 2010a;Machado et al., 2010b; Zhang et al., 2011).

The aim of the present paper is a betterunderstanding of Cr(VI) biosorption/reduction by heatinactivated yeast biomass (Saccharomyces cerevisiae),in a batch system by varying pH, biosorbent dosages,contact time and temperature, as well as to elucidatethe mechanism of metal uptake by S. cerevisiae thatrepresents a real challenge in the field of biosorption,thorough kinetic modeling of the bioremoval of heavymetal by S. cerevisiae and FTIR and SEM analysis.

MATERIALS & METHODSCommercially distributed ‘active’ wet pressed

biomass of Saccharomyces cerevisiae was suppliedfrom a local store. Biomass was washed three times indistilled water followed by centrifugation at 6,000 rpmfor 10 min. Yeast biomass was inactivated by heatingin an oven at 80 °C for 24 h (Hlihor et al., 2010). Thedried yeast was grounded in a mortar to a constantparticle size of 125-250 µm. In order to test the absenceof viability of heat inactivated biomass an additionallyprocedure was applied: the dried cells were suspendedin sterilized distilled water and incubated for one weekat 28 ºC on agar plates. After this time period, no living

colonies were found, so that it was demonstrated thatall the yeast population was dead. All chemicals usedin this work were of analytical reagent grade and wereused without further purification. Chromium stocksolution of 1000 mg/L was prepared by dissolvingK2Cr2O7 (Riedel) in distilled water. The solution wasdiluted for different metal concentrations by distilledwater as required by the working procedure. Samplesof Cr(VI) were analyzed spectrophotometrically bymeasuring the absorbance of the pink colored complexof Cr(VI) with 1,5-diphenylcarbazide in acidic solutionat 540 nm (T60 UV-Visible Spectrophotometer) asdescribed in the Standard Methods (Clesceri et al.,2005). For total Cr concentration analysis, the Cr(III)was oxidized to Cr(VI) at high temperature by theaddition of an excess of potassium permanganate priorto the 1,5-diphenylcarbazide reaction. Theconcentrations of Cr(III) were then obtained by thedifference between total Cr and Cr(VI) concentrations(Clesceri et al., 2005).

The biosorption of Cr(VI) was examined in batchin 150 mL conical flasks by mixing the desired amountof inactivated biomass with 50 mL solution of knownCr(VI) concentration and agitated at 150 rpm. A seriesof assays, with 100 mg/L Cr(VI) were conducted underdifferent pHs (1-4) and room temperature (25±1°C) toinvestigate the influence of pH on Cr(VI) biosorptionby inactivated S. cerevisiae. In the case of optimumbiomass dosage determination, 1 to 8 g/L S. cerevisiaewere used. For initial Cr(VI) concentration variationwith time, concentrations of 25, 50, 100 and 200 mg/Lwere used, and the experiments were performed atdifferent temperatures (25 ºC, 40 ºC and 50 ºC). Fordesorption experiment, the Cr-laden biomass wasobtained through contact of 50 mg/L Cr(VI) pH 2 and50 ºC for 24 h, long enough to reach 100% Cr(VI)reduction. Then the experiment was continued at 50 ºCwith 50 mL H2SO4 and NaOH (0.5 M) as eluants (Park etal., 2005). Samples were taken at predetermined timeintervals for 30 days, and analyzed for Cr(VI) and totalCr. The initial pH of the working solutions was adjustedby addition of 0.1M H2SO4 and 0.1M NaOH solutions.The change in the working volume due to the additionof H2SO4 and NaOH was negligible. The pHmeasurements were performed with a HANNA precisionpH meter, model pH 213. The batch experiments werecarried out in an orbital incubator IKA KS 4000 ICcontrol. All the experimental data were the averages ofduplicate or triplicate experiments. All glassware usedfor experimental purposes was washed in 20% nitricacid and rinsed with distilled water to remove anypossible interference by other metals.

Infrared spectra of heat inactivated S.cerevisiae biomass with and without the metal ion wereacquired using JASCO FT/IR-4200 Spectrometer. The

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powdered dried samples after the biosorption processwere analyzed in the range 3500-600/cm with aresolution of 4/cm by using the ATR technique andplacing enough sample to cover the diamond sensor.

SEM of the heat inactivated S. cerevisiaebiomass with and without the metal ion was taken witha VEGA II LSH SEM (Czech Republic). The cell samplescoated with carbon were examined under high vacuummode.

Metal uptake in time and at equilibrium, andbiosorption efficiency of heat inactivated biomass weredetermined according to Eqs. (1-3):

Vm

CCq ei

e−

= (1)

Vm

CCq ei

t−

= (2)

100(%) ×−

=i

ei

CCC

R (3)

where: qe and qt are the amount of metal removed fromsolution (mg/g) at time t and at equilibrium; R(%) isthe biosorption efficiency; Ci and Ce are theconcentrations (mg/L) of metal ion in the initial solutionand at the equilibrium after the experiment; V (L) is thevolume of the solution; m (g) is the amount of biomassused in the experiment.

Desorption efficiency was given as (Eq. 4):

Different modeling approaches are used torepresent the kinetic data of heavy metal biosorption.Distinctive adsorption kinetic models are of extensiveuse in explaining the biosorption mechanism of heavymetals (Gavrilescu, 2004; Pagnanelli, 2011). In spite ofthe importance of adsorption equilibrium studies, thatdetermine the efficacy of adsorption, kinetic modelshave been exploited on the purpose of investigatingthe mechanism of biosorption and its potential rate-controlling steps. In addition, information of thekinetics of metal uptake is required to select the bestconditions for full-scale batch metal removal process(Febrianto et al., 2009).

In order to investigate the mechanism of sorptionvarious kinetic models have been suggested and aregenerally expressed in linear form as follows:Lagergren pseudo-first order model (Smaranda et al.,2011)

tk

qqq ete 303.2log)log( 1−=− (10)

Pseudo-second order model (Ho, 2006)

tqqkq

t

eet

112

2

+= (11)

Elovich model (Gupta and Bhattacharya, 2011)

qtln)ln(q1

t ββαβ

+= (12)

where qe and qt (mg/g) are the adsorption capacity atequilibrium and at time t (min), respectively k1 is therate constant of the pseudo-first order equation (permin), k2 is the rate constant of pseudo-second orderadsorption (g/mg min), α is the Elovich initialadsorption rate (mg/g min) and β is the Elovichdesorption constant (mg/g min) during one experiment.

Some authors have recently reported that thebiosorption mechanism of Cr(VI) by biomaterials is not“anionic adsorption” but “adsorption-coupledreduction” (Park et al., 2007), therefore kinetic modelsbased on reduction were also applied to fit theexperimental data of Cr(VI) bioremoval by S. cerevisiae(Eqs. 13, 14) (Wu et al., 2010):Pseudo-first order reduction:

,3CkdtdC

−= tkCC 30lnln −= (13)

Pseudo-second order reduction:

,24Ck

dtdC

−= 0

411

Ctk

C+= (14)

where C0 and C are Cr(VI) concentrations in solution(mg/L) at time 0 and t respectively, and k3, k4, are theapparent rate constants. The kinetic models parameterswere evaluated by linear regression using ORIGIN Pro8software.

RESULTS & DISCUSSIONDue to the acidic nature of chromium-containing

aqueous real solutions from specific industries, thepH experiments were conducted in the pH range 1-4with 100 mg/L Cr(VI) and at 25 oC. Fig. 1 shows theeffect of initial pH on Cr removal at various solutionpHs, ranging from 1 to 4; pH profiles were investigatedwith no pH adjustments during the experiment. Apreliminary study (Hlihor et al., 2011) showed that in 5d, Cr(VI) was completely removed from the aqueoussolution by 5 g/L yeast biomass only at pH 1. In the

100% ×=adsorbedionsmetalofamountdesorbedionsmetalofamount

D (4)

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case of pH 2, 3 and 4 only 68.49%, 26.24% and 11.05%Cr(VI), respectively were removed from aqueoussolution during the experiment (Hlihor et al., 2011).Further experiments were conducted in order to achieve100% for all the pH values, as shown in literature forother biomaterials (Park et al., 2007; Wu et al., 2010).As expected, the removal rate was strongly pHdependent; the effectiveness of the reduction of Cr(VI)to Cr(III) increases with decreasing the pH from 4 to 1(Fig. 1a), and total Cr concentration decreased withincreasing the pH from 1 to 4 (Fig. 1b). Although Cr(VI)reached 100% removal at pH 1-2, total Cr reached 32-36% removal at this pH values and Cr(III) which initiallywas not present appeared in the solution (Fig. 1b). Heatinactivated S. cerevisiae could remove and reduce 71.8and 76.8% of Cr(VI) to Cr(III), and 62.5 and 69.6% oftotal Cr at pH 3 and 4, respectively after 90 d. Theincrease of solution pH from 1 to 4 increases thenegative charge on the cell surface due to thedeprotonation of the metal binding sites henceattracting Cr(III) ions resulting from the reduction ofCr(VI) (Silva et al., 2009). At low pH, HCrO4

- speciesare dominant and enter in reactions, when Cr(VI) isreduced to Cr(III). Depending on the pH of the solution,Cr(VI) species may be in the forms (Baral et al., 2006;Silva et al., 2009): dichromate (Cr2O7

2-), hydrochromate(HCrO4

-), chromate (CrO42-). Also, Cr(III) species may

take the following forms (Silva et al., 2009): hydratedtrivalent chromium, Cr(H2O)6

3+, Chromium hydroxidecomplexes, Cr(OH)(H2O)5

2+ or Cr(OH)2(H2O)4+. These

forms also determine the behavior at various pH valuesand intervals (Deng and Bai, 2004; Quintelas et al.,2009): as a result of repulsive electrostatic interactions,

Cr(VI) anion species are poorly adsorbed by thenegatively charged groups and can move freely; Cr(III)species, which carry positive electric charges couldbe adsorbed on the negatively charged groups. Thereactions leading to reduction of Cr(VI) to Cr(III) canbe summarized as follows (Silva et al., 2009):

Cr2O72- + 14H+ + 6e- ↔ 2Cr3+ + 7H2O (18)

CrO42- + 8H+ + 6e- ↔ Cr3+ + 4H2O (19)

HCrO4- + 7H+ + 3e- ↔ Cr3+ + 4H2O (20)

H2CrO4- + 6H+ + 3e- ↔ Cr3+ + 4H2O (21)

Taking into consideration the reduction of Cr(VI)and total Cr removal by inactivated S. cerevisiae, fornext studies, pH 2 was selected as optimum pH. Fig.2a shows the time-dependent concentration of Cr(VI)at various biomass dosages (1 to 8 g/L heat inactivatedS. cerevisiae). The removal rate of 100 mg/L Cr(VI)increased with increasing the biomass dosage. 100%reduction of Cr(VI) to Cr(III) was possible for 3, 5 and8 g/L inactivated S. cerevisiae at a contact time of 90d, 12 d and 25 d. At 8 g/L biomass dosage, the removalrate was slower than at 5 g/L; this could be explainedby the fact that the competition of the ions for theavailable sites caused a decrease of the removalefficiency with increment in the biosorbent dosage(Deng et al., 2009).After a complete removal/reductionof Cr(VI), 31.6, 36.01 and 49.03 % of total Cr was boundto the biomass (Fig 2b) at 3, 5 and 8 g/L, respectively.For 1 g/L, even if the reduction of Cr(VI) was possible

Fig. 1. Effect of solution pH on chromium reduction and biosorption from aqueous solution by heat inactivatedS. cerevisiae: a) Cr(VI) (mg/L) vs. time, b) Total Cr (mg/L) and Cr(III) (mg/L) at the end of contact time

(biomass dosage: 5 g/L; Cr(VI) concentration: 100 mg/L; temperature: 25 oC)

Bioremediation of Cr(VI) Polluted Wastewaters

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Fig. 1. Effect of solution pH on chromium reduction and biosorption from aqueous solution by heat inactivatedS. cerevisiae: a) Cr(VI) (mg/L) vs. time, b) Total Cr (mg/L) and Cr(III) (mg/L) at the end of contact time

(biomass dosage: 5 g/L; Cr(VI) concentration: 100 mg/L; temperature: 25 oC)

Fig. 2. Effect of biomass dosage on chromium reduction and biosorption from aqueous solution by heatinactivated S. cerevisiae: a) Cr(VI) (mg/L) vs. time, b) Total Cr (mg/L) and Cr(III) (mg/L) at the end of contact

time (pH 2; Cr(VI) concentration: 100 mg/L; temperature: 25 oC)

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57.7%, the total Cr removal reached at the end ofcontact time (90 d), 11.82 %. The sorption on thesurface is saturated faster at low biomass dosages.The availability of higher energy sites decreases witha larger fraction of lower energy sites occupied,resulting in a lower uptake value and higher removalefficiency at higher biomass dosages (Gupta andRastogi, 2008). In further studies, pH 2 and 5 g/Linactivated S. cerevisiae were used for the reductionof Cr(VI) and total Cr removal from aqueous solution.The concentration of Cr(VI) versus time was examinedat various initial Cr(VI) concentrations, from 25 to 200mg/L and at different temperatures, 25, 40 and 50 oC;pH 2, 5g/L biomass dosage and 150 rpm were maintainedconstant. At 25 oC, the complete removal of 25 mg/LCr(VI) required 8 d, while 200 mg/L Cr(VI) achieved aremoval of 41 % in 17d (Fig. 3a). The increase oftemperature greatly increased the Cr(VI) removal rateand decreased the contact time required for a completeremoval of Cr(VI) (Fig. 3b, c). The necessary time for acomplete removal of Cr(VI) at 50 oC was 2d and 17d for25 and 200 mg/L, respectively. As mentioned in similarstudies employing biomaterials, the increase oftemperature induces the rate of the redox reaction (Parket al., 2005). Cr(VI) could be completely removed fromsolution as long as the contact time was sufficient andwith rise of the temperature; the mechanism involved inCr(VI) removal by inactivated S. cerevisiae is thereforenot ‘‘anionic adsorption” but ‘‘adsorption coupledreduction”, as shown in previous works employingbiomaterials (Park et al., 2006; Deng et al., 2009).

Because both adsorption and reduction couldhave an important impact in the removal of Cr(VI) fromaqueous solution, the kinetic models based onadsorption and reduction were analyzed and theregressions parameters are reported in Table 1.It isindicated that for the experimental data regarding theinfluence of initial Cr(VI) concentration andtemperature (25 and 50 ºC) (Fig. 4), the best fitting modelis pseudo-first-order reduction model; in contrast, forthe experiments performed at 40 ºC the best fittingmodel is pseudo-first-adsorption model.Theexperiments regarding the influence of pH and biomassdosage were best fitted by the pseudo-second-orderreduction model. These results are indicating possibledifferences in the Cr(VI) removal mechanism at differentexperimental conditions. The values of R2 indicatedfor the best fitting kinetic models are higher than 0.9.Based on the assumption that a biosorbent promotesan economic treatment of aqueous solutions, thepossibility of metal recovery was studied throughdesorption experiments. Desorption of Cr bound tothe biomass seems to be a difficult task consideringthe assumption that Cr(VI) was reduced to Cr(III).Several researchers noticed that solutions of alkali and

highly alkaline salts (NaOH, Na2CO3 and NaHCO3) weremore potent than acids or mineral salts for Cr (VI)desorption (Isa et al., 2008).

The desorption experiments were studied using0.5M H2SO4 and 0.5M NaOH as eluants (Fig. 5a, b) at50 °C, in batch. As seen in the fig, the 0.5 M NaOH is amore suitable eluant then H2SO4, but with a lowdesorption efficiency (12 %) of the Cr bound to thebiomass in 30 d. 0.5 M H2SO4 eluted only the Cr(III),while both Cr(VI) and Cr(III) were eluted by 0.5M NaOH.In this case, Cr(III) appeared only until 150 min anddisappeared from the solution slowly after 240 min.Based on this results, it is possible that both Cr(III)and Cr(VI) exist on the biomass and the latter might bereduced to the former which was eluted into theaqueous solution (Park et al., 2005). This studysuggests that a higher concentration of NaOH may berequired to increase desorption of possible Cr speciesbound on S. cerevisiae biomass. A further study wasnot performed, since a more concentrated NaOHsolution would raise the cost of a wastewater treatment,putting under discussion the suggested low-costtechnology.

FTIR and SEM analysis were used as techniquesfor biosorbent characterization, as well as to explorethe possible mechanism involved in the removal ofCr(VI) by S. cerevisiae. The FTIR spectrum of dried S.cerevisiae before and after biosorption of Cr(VI) isshown in Fig. 6. The unloaded yeast biomass, presentsa broad band in the region 3750-3230/cm, with amaximum at 3277.4/cm; this peak is present due tohydroxyl (OH) stretching vibrations bonded with N-Hstretching. Methylene C-H stretching bands typicallyassigned at 2940 – 2915/cm are present with a peak at2924.5/cm. The peaks at 1638.2/cm and 1535.05/cm arethe result of the stretching vibrations of C=O (carbonyl)and N-H(amide) deformation bands which result in IRbands between 1900-1550/cm and 1660-1500/cm,respectively. The band at 1396.2/cm results in a bendingvibration of COOH and OH groups (Dai et al., 2008).The P=O stretching vibration occurs at 1040.4/cm. Thephosphate out-of-phase stretch has strong IR bandsbetween 1180 and 1000/cm.

After the retention of metal ion on the biomass,changes in intensity and shift in the peaks positionoccur. This indicates that heavy metal removal processby S. cerevisiae biomass doesn’t imply the formationof stable covalent bonds, but is predominantly realizedby chemical interactions (ion-exchange). Chromiumions have strong interactions with functional groupsfrom peaks 1515.7/cm (C=O (carbonyl) and N-H(amide)deformation bands) and 1036.5/cm (P=O stretching).In these regions, there is an enhancement in intensity,due to chromium interaction.

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Fig. 3. Effect of contact time on Cr(VI) reduction by heat inactivated S. cerevisiae at different temperatures: a)25 ºC; b) 40 ºC; c) 50 ºC (pH 2; biomass dosage: 5 g/L)

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Bioremediation of Cr(VI) Polluted Wastewaters

Table 1. Kinetic parameters obtained for Cr(VI) removal by heat inactivated S. cerevisiae

Pseudo-first order Pseudo-second order Kinetic models Item Value

k R2 k R2

25 0.0125 0.9016 0.0027 0.5728 50 0.0196 0.9559 0.0021 0.7120 100 0.0148 0.9763 0.0014 0.9271

Cr(VI) concentration (mg/L)25 °C

200 0.0017 0.833 7.27 10-6 0.7788 25 0.0171 0.8456 0.0051 0.8901 50 0.0125 0.7226 0.0021 0.8716 100 0.0081 0.9541 4.88 10-4 0.8608

Cr(VI) concentration (mg/L) 40 °C 200 0.0038 0.9329 5.85 10-5 0.9330

25 0.0016 0.3225 0.0012 0.3259 50 0.0418 0.901 0.0134 0.7768 100 0.0153 0.9600 0.0017 0.9254

Cr(VI) concentration (mg/L) 50 °C

200 0.0087 0.9948 4.37 10-4 0.8682 1 0.0414 0.9339 0.0046 0.8876 2 0.0148 0.9763 0.0014 0.9271 3 5.55 10-4 0 .8871 1.14 10-5 0.9769 pH

4 6.40 10-4 0 .8817 1.50 10-5 0.9697 1 3.40 10-4 0 .9089 5.45 10-6 0.9599 3 0.0012 0.9237 3.39 10-5 0.9271 5 0.0148 0.9763 0.0014 0.9271

Reduction kinetics

biomass dosage (g/L)

8 0.0126 0.9594 0.0014 0.9296 25 0.0283 0.7624 0.0137 0.5728 50 0.0147 0.9232 0.0102 0.8671 100 0.0149 0.9754 4.99 10-4 0.9493

Cr(VI) concentration (mg/L)25 °C

200 0.0075 0.8865 0.0011 0.9439 25 0.0138 0.9269 0.0258 0.8901 50 0.0118 0.8011 0.0124 0.8372 100 0.0070 0.9753 0.0024 0.8608

Cr(VI) concentration (mg/L)40 °C

200 0.0054 0.9360 6.07 10-4 0.9719 25 0.0016 0.3225 0.0063 0.3259 50 0.0418 0.901 0.0671 0.7769 100 0.0153 0.9600 0.0089 0.9254

Cr(VI) concentration (mg/L)50 °C

200 0.0087 0.9948 0.0021 0.8682 1 0.0414 0.9339 0.0234 0.8876 2 0.0149 0.9754 4.99 10-4 0.9493 3 0.0021 0.9586 2.52 10-4 0.9186 pH

4 0.0023 0.8583 2.73 10-4 0.7956 1 0.0021 0.7574 5.42 10-5 0.8140 3 6.05 10-5 0 .6750 3.70 10-5 0.7405 5 0.0149 0.9754 4.99 10-4 0.9493

Adsorption kinetics

biomass dosage (g/L)

8 0.0037 0.9416 0.0046 0.9250

Fig. 7 presents the SEM micrographs of heatinactivated S. cerevisiae before (Fig. 7a) and afterchromium binding (Fig. 7b). The inactivated biomassshowed a porous structure and heterogeneousmorphology, with pores with both larger and smallerdimensions. The yeast cells were oval and their surfaceappears to be smooth. The SEM micrograph of Cr-loaded yeast, indicates that the surface morphologydoesn’t change much; this leads to the conclusion

that Cr(VI) reduction occurs at the interface of theadsorbent. FTIR and SEM analysis confirm that Cr(VI)can be directly reduced to Cr(III) in aqueous phase bycontact with the electron-donor groups and that therelease of the Cr(III) ions into the aqueous phase isdue to electronic repulsion between the positivelycharged groups and the Cr(III) ions, or due tocomplexation of Cr(III) with adjacent groups capableof Cr-binding.

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Fig. 4. Pseudo-first order reduction model applied for the removal of Cr(VI) at different temperatures by heatinactivated S. cerevisiae: a) 25 ºC b) 50 ºC

Fig. 5. Desorption profiles of chromium bound on the yeast biomass by different eluants: a) H2SO4 0.5M; b) NaOH 0.5M

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Fig. 5. Desorption profiles of chromium bound on the yeast biomass by different eluants: a) H2SO4 0.5M; b) NaOH 0.5M

Fig. 6. FTIR spectra of: a) heat inactivated S. cerevisiae, b) heat inactivated S. cerevisiae loaded with chromium

Hlihor, R.M. et al.

Fig. 7. SEM micrographs of: a) heat inactivated S. cerevisiae, b) heat inactivated S. cerevisiae loaded with chromium

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CONCLUSIONSThe inexpensive S. cerevisiae yeast, produced in

large quantities in the food and beverage industrycould successfully remove Cr(VI) from aqueoussolutions. Chromium reduction and biosorption washighly pH dependent, 100 mg/L of Cr(VI) beingcompletely removed at pH 1-2 in 5-12 days by 5 g/Lyeast biomass. Adsorption and reduction models wereapplied to the experimental data of the removal of Cr(VI)from aqueous solution. The experiments regarding theinfluence of pH and biomass dosage follow the pseudo-second-order reduction model while the experimentsregarding the influence of initial Cr(VI) concentrationand temperature (25 and 50 ºC) follow the pseudo-first-order reduction model. Results based onexperimental data and FTIR and SEM analyses revealedthat Cr(VI) can be directly reduced to Cr(III) in aqueousphase by contact with the electron-donor groups ofthe biomass release and the converted Cr(III) can beadsorbed to various functional groups of the biomassdepending especially of the pH values. It can beconcluded that the heat inactivated S. cerevisiae is aneffective and alternative biomass for the removal ofCr(VI) form aqueous solutions, especially because itrepresents a by-product of fermentation industry,being produced in large quantities.

ACKNOWLEDGEMENTThis paper was elaborated with the support of

BRAIN project Doctoral scholarships as an investmentin intelligence - ID 6681, financed by the EuropeanSocial Found and Romanian Government and with thesupport of a grant of the Romanian National Authorityfor Scientific Research, CNCS – UEFISCDI, projectnumber PN-II-ID-PCE-2011-3-0559", Contract 265/2011.

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